An anti-police violence slogan online has been turned on its head, and with a slight change it's being used in support of two officers shot in Ferguson.

In the hours after two police officers were shot in Ferguson, Missouri, use of the hashtag #BlueLivesMatter spiked on Twitter, driven by self-identified supporters of gun rights and other conservative causes, as well as supporters of the police. The tag was used across the US, not just in Missouri.

Although the overall numbers are relatively low - the tag has so far been used more than 3,000 times since news of the shooting broke - the slogan hit the site's list of top trends in the US because of how quickly it spiked. But the use of "Blue Lives Matter" is not new. In fact it's been bubbling along at a low level for months. It's been used more than 120,000 times since last November, when radio talk show host @BarryGadbois tweeted: "Cops have a right to defend themselves and an obligation to protect you. #BlueLivesMatter #Ferguson".

Among the pro-police messages, however, were a scattering of anti-police tweets. "They're victims of the environment THEY created," commented New Yorker @AfiaBlack. "#BlueLivesMatter, of course, but rarely does anyone who shoots a cop go unpunished. Equal justice needs 2 become a thing. #BlackLivesMatter," tweeted @laughingliberal.